In an image-forming apparatus of an electrophotographic system, by applying an image-forming process of electrostatically charging, light-exposing, and developing to an electrophotographic photoreceptor of a rotary drum-type, etc., to form an image, and after transferring the image onto a transfer material, the image is fixed to obtained a copy and as such an image-forming apparatus, there are, for example, a plain paper copying machine (PPC), a laser printer, a light emitting diode (LED) printer, a liquid crystal printer, etc. As the electrophotographic photoreceptor used for the apparatus, an inorganic type electrophotographic photoreceptor comprising selenium, arsenic-selenium, cadmium sulfide, zinc oxide, etc., has been used. On the other hand, the investigation and development of organic type electrophotographic photoreceptor which are inexpensive and excellent in the points of the productivity and the waste disposal have been actively made and among them, a so-called function-separation type laminated electrophotographic photoreceptor having the laminate of a charge generating layer and a charge transport layer is excellent in the point of the electrophotographic characteristics such as the sensitivity, the charging property, and repeating stability, etc., and various such electrophotographic photoreceptors have been proposed and practically used.
Recently, with the increase of the performance of electrophotographic photoreceptors, high-speed copying machines and printers have been used. Furthermore, with the propagation of computers, the needs of so-called desk top publishing have been increased and small-sizing of the machine itself and small-sizing of an electrophotographic photoreceptor accompanied thereby have been strongly desired.
To obtain a stable image, it is necessary to remove the surface charge on the electrophotographic photoreceptor during the time of after light-exposing the image on the electrophotographic photoreceptor and before initiating the development. The requirement for the high speed and small-sizing is, in other words, shortening of the time required for removing the surface time, that is, the response time. If the response time is not sufficiently quick, breaking of images and thinning of fine lines occur, which becomes a large problem in a color image-forming machine requiring a particularly severe color reproducibility. The response time is dominated by the charge mobility (.mu.) in the charge transport layer and the mobility is defined by following formula (1) from the thickness L (cm) of the charge transport layer, the voltage V (V) applied to the charge transport layer, and the time t.sub.T (second) required for a carrier reaching another surface of the charge transport layer from one surface of the transport layer: EQU .mu.=L.sup.2 /V.multidot.T.sub.t (1)
Because the charge mobility is dominated by the charge transporting material and the molecular structure of a binder resin in the transport layer, vigorous investigations have been made about these materials for increasing the charge mobility. As the results thereof, as the effective means for increasing the charge mobility, the following matters (a) to (c) have been clarified.
That is, (a) the charge transporting material has many phenyl groups capable of conjugating with a nitrogen atom, has a large extension of a conjugated system, and does no cause the deviation of the charge in the molecule (for example "Densishahsin Gakkai Shi (Journal of Electrophotographic Society)", 25(3), 16(1986); "Journal of Electrophotographic Society", 29(4), 366(1990); Journal of Applied Physics", 69, 821(1991), etc.).
(b) The binder resin does not have a polar group forming a trap of a carrier (for example, "Journal of Electrophotographic Society", 64th Investigation Forum, 75(1989); "Philosophical Magazine, Lett.", 62(1), 61(19990), etc.), etc.
(c) The concentration of the charge transporting material in the charge transport layer is increased (for example, "Journal of Applied Physics", 43(12), 5033(1972), "Journal of Electrophotographic Society", 25(3), 16(1986), etc.).
As the results of these investigations, practical materials as shown below have been investigated. That is, as the charge transporting material, a high-charge-mobility charge transporting material such as triarylamine, tetraarylbenzidine, stylbene, etc., and as the binder resin, styrene, polyphenylene oxide, polycarbonate, etc., have been developed and practically used. Also, as a means of increasing the concentration of the charge transporting material in the charge transport layer, charge transporting polycarbonate, polyester, polysilane, etc., obtained by polymerizing each charge transporting component of a charge transporting material have been vigorously investigated as the effective means. However, these charge transporting materials have been practically used for copying machines and printers but have not yet sufficient for the needs of further increasing the speed and further small-sizing.
At present, an electrophotographic photoreceptor of a so-called lamination type, that is, composed of a laminate of laminating a charge transport layer on a charge generating layer has become the mainstream and thus generally the charge transport layer becomes a surface layer. However, in a low-molecule dispersion type charge transport layer which is the mainstream at present, the charge transport layer having a sufficient performance in regard to the electric characteristics has been obtained but because a low molecular material is dispersed in a binder resin, there is a fault that the mechanical properties essential to the binder resin are lowered and the layer is essential weak in regard to abrasion. Accordingly, the amount of the charge transporting material dispersed in the binder resin is practically at most from 45 to 50% by weight. Also, when polystyrene, polyphenylene oxide, a polyphenylenevinylene derivative, etc., is used as the binder resin, the charge mobility can be increased even when the amount of the charge transporting material is same but these resins are inferior in the mechanical strength to polycarbonate and a polyester resin which have been practically used. Consequently, from various practical problems, there also occurs a limit in the response time and in the case of using for the process wherein the time from light-exposure to development is generally 150 m sec., or shorter, particularly 120 m sec., or shorter, and more particularly 100 m sec., or shorter, a problem becomes obvious. From the special demand of the circumference of an electrophotographic photoreceptor, the condition becomes remarkable when the diameter of the electrophotographic photoreceptor is not larger than 40 mm, particularly not larger than 30 mm, and more particularly not larger than 25 mm. Also, a contact charging system which has been practically used as a charging method of being reluctant to generate ozone for the consideration to an environmental problem, particularly, a contact charging system wherein the charging voltage has an alternating current component caused an abrasion acceleration of at least from 5 to 10 times as compared with a non-contact charging system such as Corotron, etc., and the problem of the durability of the electrophotographic photoreceptor becomes more remarkable. The more increase of the copying speed and more small-sizing of an electrophotographic photoreceptor means that the number of the repeated use of an electrophotographic photoreceptor per unit time is increased and in the performance of an electrophotographic photoreceptor of the present, the exchange of an electrophotographic photoreceptor becomes inevitable in a short time, which becomes a large cost up. Moreover, it is necessary to hasten the response time, as a means for increase the mobility for the purpose, it is most actual to increase the charge transporting material in the charge transport layer, but the increase of the amount of the charge transporting material is accompanied by more lowering the mechanical strength of the charge transport layer, which caused a problem in practical use.
On the other hand, a method of incorporating at least 54% triarylamine or the derivative thereof in a charge transport layer for the purposes of improving the photosensitivity and the response time is disclosed in Japanese Patent Laid-Open No. 53339/1993. However, the method aims at the prolong of the life of the charge transport layer by increasing the thickness of the layer as much as the improvement of the response time or at the prolong of the life by increasing the thickness of the charge transport layer and further forming thereon a protective layer, and does not aim at positively shortening the response time.
Also, a surface protective layer formed by dispersing an electrically conductive fine powder in an insulating resin is well known. However, this is for controlling the resistance by controlling the dispersed amount of the electrically conductive fine powder and in the above-described case, it is difficult to control the direction of flowing of electrostatic charges and essentially, image flowing is liable to occur. Because the flow of electrostatic charges has a time reliance and diffuses with the passage of time, the image flowing severely occurs when the time of from a light-exposure to a development is long. Accordingly, in the case of using the electrophotographic photoreceptor the life of which is prolonged by forming a surface layer having a high mechanical strength for an electrophotographic apparatus having a long time from a light-exposure to a development, that is, of a low-speed process, there is a problem that the image flowing is liable to occur.
Also, it is known that polysilane is a material having a high charge mobility, but because the material has a low mechanical strength, there is a problem for practical use.